1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) device and particularly to an improvement in light emission efficiency using a thermally activated delayed fluorescence (TADF) material.
2. Description of the Related Art
An organic EL device is generally referred to as OLED (organic light-emitting diode), which is a kind of light-emitting diode. In the emissive layer of the organic EL device, a light-emitting dopant is excited by the recombination of holes injected from the anode and electrons injected from the cathode, and a singlet excited state and a triplet excited state are generated at a ratio of 1:3. In the organic EL device using a fluorescent material as the light-emitting dopant, only the singlet excited state contributes to light emission and light is not emitted when the triplet excited state is deactivated. Therefore, the limit of its internal quantum efficiency is considered to be 25%. Research has been done utilizing a TADF mechanism as an organic EL light emission mechanism to solve this problem. This TADF mechanism utilizes the phenomenon of reverse intersystem crossing (RISC) from a triplet excited state with lower energy to a singlet exciton with higher energy, generated by thermal activation in a material with a small difference in energy between the singlet excited state and the triplet excited state. According to this, theoretically, the internal quantum efficiency of fluorescent emission can be increased to 100%.
Recently, a TADF material which enables light emission in all of red (R), green (G), and blue (B) at room temperature has been developed. FIG. 10 is a schematic view for explaining a fluorescent emission mechanism in an organic EL device using a TADF material as an assistant dopant (see H. Nakanotani, et al., “High efficiency organic light-emitting diodes with fluorescent emitters,” Nature Commun. 5, 4016 (2014)). To the emissive layer of this organic EL device, the TADF material is added as well as a host material and a light-emitting dopant material. In FIG. 10, the energy level of each material is shown. So indicates the ground state. S1 indicates the lowest singlet excited state. T1 indicates the lowest triplet excited state. In the illustration, the higher the position is, the higher the energy level is. The light-emitting dopants TBPe, TTPA, TBRb, and DBP emit blue, green, orange, and red fluorescent lights, respectively. The shorter the light emission wavelength of the material is, the higher the S1 energy level is. 25% of the recombination of holes (h+) and electrons (e−) results in the S1 level of TADF molecules of the assistant dopant, and 75% results in the T1 level. Here, the TADF molecules of the T1 level are upconverted to the S1 level by the RISC process with thermal energy. Using the TADF molecules having a higher S1 level than the light-emitting dopant, as the assistant dopant, energy transfer of the singlet exciton of the TADF molecules to the light-emitting dopant of each color can be performed by fluorescence resonance energy transfer (FRET), and fluorescent emission of each color can thus be achieved.